A prosthodontist is a dental specialist who is skilled in the replacement of missing teeth and the restoration of natural teeth. A prosthodontist has graduated from dental school and usually will have three or more years of continuing education after that.
This type of dental specialist is trained to deal with complicated and simple restorations of the whole mouth as well as treating facial deformities. Common procedures treated by a prosthodontist may include dentures, partial dentures, fixed bridges, crowns, implants, veneers and more.
In short- A prosthodontist goes for three years of additional schooling to specialize in WHOLE mouth restoration, treating facial deformities, affixing dentures, bridges, crowns, implants, etc.
For more information on Prosthodontists, you can visit the Wikipedia article located here.